An antenna array is a group of multiple active antennas coupled to a common source or load to produce a directive radiation pattern. Usually, the spatial relationship of the individual antennas also contributes to the directivity of the antenna array. A phased array antenna is an array of antennas in which the relative phases of the signals feeding the antennas are varied in a manner that the effective radiation pattern of the entire array is reinforced in a desired direction and suppressed in undesired directions.
FIG. 1 shows a diagram of a known antenna array 100. The antenna array 100 includes several linear arrays 104 housed in a (non-metallic) radome 102. Here, each linear array 104 is arranged vertically with spacing between each other, which is determined by the desired resonant frequency of the antenna array 100. Each linear array 104 is connected to an associated radio frequency (RF) electronics circuit contained in an external RF electronics module 108, via an antenna feed 106. The RF electronics module 108 is connected to external systems via a connection 110 for power, control, and communications connections; and may be physically mounted on the radome 102, or may be located remotely or outside of the antenna array 100. Typically, the received electrical signal from each antenna element is digitized by a dedicated analog-to-digital converter (ADC) or the received electrical signals from multiple antenna elements are weighted and summed in analog domain prior to digitization by an ADC.
Digital beamforming is a signal processing technique used in sensor and communications applications for directional signal transmission or reception. Digital beamforming is attained by combining elements in a phased array in such a way that signals at particular angles of arrival experience constructive coherent combining, while other signals experience destructive non-coherent combining. Digital beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity. An advantage of digital beamforming is the ability to simultaneously form multiple transmit or receive beams with different weighting/shaping and leverage other signal processing techniques that can be performed in the digital domain
Modern communications and sensor (e.g., radar) payloads are constantly pushing more complexity into the digital domain. Examples of complex processing in the digital domain include channelization, wideband beamforming, frequency hopping, space-time adaptive processing (STAP), and regenerative processing. Communication systems increasingly utilize large numbers of highly directive narrow spot beams with dynamic steering to increase aggregate system throughput. Radar or sensor systems utilize narrow beams to increase angular resolution in imaging and detection applications. Such systems require a large number of antenna elements for an array-based antenna. Digital domain processing requires the input signal of each antenna element to be digitized with a dedicated ADC on the receive side and converted to analog domain with a dedicated digital-to-analog converter (DAC) on the transmit side.
Furthermore, future payloads will use direct RF sampling to enable high-performance flexible processing, requiring converters to operate at very high sampling rates. Next generation digital payloads will therefore contain many high-rate ADCs and DACs which translates to high power consumption and the associated size and weight challenges. The resulting increase in size, weight and power (SWaP) makes it prohibitive to have a high-speed converter dedicated to each antenna element.
Prior attempts at digital beamforming utilize either lower rate converters (e.g., ADC) or sub-array digital processing where signals at the antenna elements are weighted and summed in analog domain, prior to digitization. Using low rate converters results in decreased flexibility in terms of supported frequency and bandwidth requirements, while sub-array digital processing restricts the scan range and reduces the beam performance in both directivity and steerability.
Code division multiplexing (CDM) can be used to reduce the number of ADCs while maintaining the ability to digitize each antenna element for digital beamforming. FIG. 2 shows a block diagram of a conventional receiver with CDM. As known in the art, code division multiple access (CDMA) is an example of multiple access, where several transmitters can send information or several receiver can receive information simultaneously over a single communication channel. This allows several users to share the same spectral frequency bands. To prevent interference between the users, CDMA typically employs spread spectrum technology and a special coding scheme where each transmitter is assigned a unique code. In this application, CDM is used to multiplex signals from multiple antenna elements where each element is assigned a unique code. Referring now to FIG. 2, signals received on L antenna elements 2020-202L are amplified by, for example L low noise amplifiers (LNA) 2040-204L-1 and coded by L mixers 2060-206L-1 using unique codes for code division multiplexing 2080-208L-1, before being summed (combined) by a summer 210 and then digitized by an ADC 212, where L is the number of antenna elements, which is also equal to the number of unique codes.
The output of the ADC 212 is then demultiplexed using L code correlators 2140-214L in time domain. Each demultiplexed signal is then channelized into N subbands using a polyphase filter bank 2160-216L-1 and L fast Fourier transform (FFT) circuits 2180-218L-1, where L and N are integers greater than 1. A routing matrix 220 routes the L channelized signals to N beamforming circuits 2220-222N-1. Either partial or full beamforming is performed using L elements for each subband by the N beamforming circuits 2220-222N-1. In this conventional receiver, beamforming is performed at the subband level to mitigate wideband dispersion.
As shown, conventional approaches typically demultiplex each element immediately after the ADC. However, this requires an independent processing path for each antenna element, which results in significant cost in required hardware resources. As described above, in the application of Direct Sequence Spread Spectrum Code Division Multiple Access (DSSS-CDMA), one will apply a code in time domain to demultiplex a signal. As described earlier, demultiplexing early in the processing chain can lead to significantly higher complexity from a hardware standpoint.
FIG. 3 is an exemplary block diagram of a conventional transmitter with CDM. As depicted, either partial or full beamforming is performed using L elements for each subband by the N beamforming circuits 3020-302N-1. Beamforming is performed at the subband level to mitigate wideband dispersion. A routing matrix 304 routes N subband signals to L inverse fast Fourier transform (FFT) circuits 3060-306L-1. For each of the L elements, subbands are recombined into a wideband signal using IFFT circuits 3060-306L-1 and a polyphase filter bank 3080-308L-1. The recombined wideband signal from each of the L elements is coded with a unique code (c0(n)-cL-1(n)), using a multiplier circuit 3100-310L-1 and then aggregated by a summer 312 in time domain to form a combined signal, before being converted to analog signal by a DAC 314. The aggregated signals are then demultiplexed using code correlator 3160-316L-1, before being sent to respective element RF chain, including low pass filters 3180-318L-1, power amplifiers 3200-320L-1 and antenna elements 3220-322L-1.
As explained above, the typical approach of implementing CDM is to apply the unique codes in time domain. In a receiver scenario, the aggregated and code multiplexed signal fans out to L paths such that a unique code can be applied to demultiplexed each of the L antenna elements. Each path includes an intermediate processing step, which is generically defined to be some non-element specific processing. The signal is then sent to a subbanding channelizer, utilizing a polyphase filter and FFT circuit. The channelizer outputs all the subbands spanning the wideband frequency. In a transmitter scenario, L subbanded signals are first processed by a recombiner, utilizing an IFFT circuit followed by a polyphase filter. The result is sent to some non-element specific intermediate processing before applying the unique code to each of the L paths and aggregating the L paths back into a single combined waveform. However, using the conventional time domain approach becomes costly in hardware since each of the L paths has its own dedicated signal processing chain.